# withUnsafeBytes(_:) Call a function and pass a buffer representing the value of this instance’s [`attachableValue`](/documentation/Testing/Attachment/attachableValue-2tnj5) property to it. ``` borrowing func withUnsafeBytes(_ body: (UnsafeRawBufferPointer) throws -> R) throws -> R ``` ## Parameters `body` A function to call. A temporary buffer containing a data representation of this instance is passed to it. ## Return Value Whatever is returned by `body`. ## Discussion> Throws: Whatever is thrown by `body`, or any error that prevented the > creation of the buffer. The testing library uses this function when saving an attachment. This function calls the [`withUnsafeBytes(for:_:)`](/documentation/Testing/Attachable/withUnsafeBytes(for:_:)) function on this attachment’s [`attachableValue`](/documentation/Testing/Attachment/attachableValue-2tnj5) property. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)